The Proper Use of 47-mm Flat Disc Filters in Filter Sizing Studies

Filters of small areas are useful as models for larger operations such as batch-scale processing. Flat disc 47-mm microporous filters are often used for this purpose, especially in flow decline (throughput) studies aimed at sizing the effective filter area (EFA) required for large-scale production.

In the flow decline method of filter sizing, also known as the flow decay or total throughput method, the quantity of effluent produced using a given small filter area is extrapolated to determine the filter area needed to process the drug volume of an entire production batch. It has customarily been considered convenient and economically useful, both in terms of effort and material costs, to conduct such sizing studies with 47-mm flat discs, because their small size minimizes the quantity of fluid involved and the operational time required for the assay.

Figure 1. Variation in unspecific adsorption (mg per 10-inch cartridge) when different filter designs are used with the same filter polymer

If desired, however, discs of an even lower EFA may be used. The smaller the assay filter, the less product is consumed in the testing. This can be an important consideration if the fluid being tested is an expensive drug. Because pharmaceutical filtration is a technico-economic enterprise, the expenses involved are reflected in the drug's cost of goods.

Regrettably, extrapolating from data obtained by using smaller filter areas or over shorter test intervals tends to give less dependable results. The smaller the filter area and liquid volume used, the less accurate the extrapolation. Consequently, the use of 47-mm flat disc filters in flow decay studies sometimes produces results that are so inexact that they must be used with an extremely high safety margin. Such allowances usually are set at 15–20%, but can be as high as 150%. Such over-sizing will result in high value losses because of unnecessary large hold-up volumes and unspecific adsorption. In such circumstances, the savings achieved from the flow decay studies are lost because of batch-to-batch running costs. For example, a 10-inch filter element could adsorb an average of 200 mg of drug (Figure 1), which commonly can be doubled to 400 mg for a 20-inch filter, which could be required if the filter were improperly scaled. The per-batch value of such waste can be calculated from the dosage and value of the drug product being filtered.1,2 The importance of proper filter sizing also has received attention in the revised PDA Technical Report #26, which describes filter choice and trials in detail.3